The invention relates to a DC/DC converter operating on the principle of a charge pump, comprising at least one charge pump capacitor and several controllable switches connected thereto which are actuated during charge pump operation by a control circuit with an oscillator to generate across a storage capacitor applied to the output of the converter an output voltage deviating from the input voltage of the converter and a comparator which compares a reference voltage to a voltage proportional to the output voltage of the converter and outputs a control signal for the control circuit, the control signal comprising a first level when the output voltage drops below a predetermined design value and comprising a second level when the output voltage exceeds the design value, the control circuit signalling the charge pump ON when the control signal changes from the second to the first level and signalling the charge pump OFF when the control signal changes from the first to the second level.
Many electronic circuits require in addition to the supply voltage further voltages sometimes at a level above that of the supply voltage. One low-cost, simple and with respect to converters employing a coil highly compact solution for making these further voltages available are DC/DC converters operating on the charge pump principle. Converters of this kind are described e.g. in the text book xe2x80x9cThe Art of Electronicsxe2x80x9d by Paul Horowitz, 2nd edition, Cambridge University Press, New York, 1991 on pages 377 to 379 thereof.
Horowitz also describes a simple DC/DC converter operating on the charge pump principle which may be employed for substantially doubling an input voltage. The basic circuit of the converter consists substantially of a charge pump capacitor and four controllable switches (e.g. MOSFETs), whereby one electrode of the charge pump capacitor is connectable via a first switch to the input voltage terminal of the converter and via the second switch to ground, and the other electrode of the capacitor is connectable via the third switch to the input voltage terminal and via the fourth switch to the output voltage terminal of the converter. The converter comprises further control circuit with a clock oscillator which signals the switches so that in a first phase of a clock cycle, the so-called charge phase, the second switch and the third switch are ON whilst the other switches are OFF so that the charge pump capacitor is charged to the input voltage, and in a second phase of a clock cycle, the so-called discharge phase, the first switch and the fourth switch are ON whilst the other switches are OFF so that the charged charge pump capacitor is then connected in series to the input voltage, resulting in a voltage value at a smoothing and storage capacitor located at the output of the circuit corresponding to roughly twice the input voltage.
Correspondingly, charge pumps are conceivable which generate any multiple of the input voltage, invert or reduce the input voltage.
In the DC/DC converter described, operating in accordance with the charge pump principle, the output voltage undesirably drops off sharply at even small load currents, however. Since in the majority of applications the output voltage, which e.g. in digital electronic circuits is often 3.3 or 5 V, is fixed and is allowed to fluctuate only in a tight range, regulated converters have been developed which set the output voltage to a fixed, desired voltage value.
The regulators of the DC/DC converters comprise as a rule a comparator which compares the actual output voltage or a voltage proportional to the actual output voltage (the proportional voltage being derived e.g. from the output voltage via a voltage divider) to a predetermined reference voltage representing the design output voltage and then, when a deviation is sensed, outputs a control signal, with the aid of which the actual output voltage is adapted to the predetermined design output voltage value.
Described in U.S. Pat. No. 5,680,300 are two types of regulators employing DC/DC converters operating on the charge pump principle, i.e. the so-called linear regulator and the so-called skip mode regulator.
In the linear regulator the control signal of the comparator changes, e.g. via a gate of one of the MOSFET switches, the ON resistance of the MOSFET so that the drop in voltage across the switch is increased or decreased, resulting in a reduction or increase in the actual output voltage. The linear regulator has, however, the drawback that the losses occurring on switching the charge pump switches are relatively large since the charge pump in the case of the linear regulator is always in operation; it thus being suitable only for relatively large load currents occurring continually.
These drawbacks do not occur in the so-called skip mode regulator since it uses the control signal of the comparator to signal the charge pump alternatingly ON/OFF so that a charge is pumped to a smoothing and storage capacitor located at the output of the circuit only when the voltage connected thereto has dropped below the design output voltage level. The skip mode regulator thus operates in a particularly energy-saving way and is especially suitable for applications in which sometimes small and sometimes large load currents may occur.
For illustration, FIG. 1 shows a prior art DC/DC converter incorporating a skip mode regulator which combines the cited Horowitz charge pump arrangement with the skip mode regulator as described in U.S. Pat. No. 5,680,300. One such converter is also described as type MAX679 in a product catalog of the Company Maxim Integrated Products available in May 1999 under the site address http://www.maxim-ic.com.
However, such existing converters incorporating a skip mode regulator have the drawback that the output current of the charge pump comprises relatively large current spikes during operation of the converter under normal load conditions. The reason for this is that the charge pump needs to be designed so that it is also capable of furnishing a predetermined minimum output current even when its working conditions deteriorate, for instance, due to a drop in the input voltage, an increase in temperature or poor process quality MOSFET switching transistors.
These high current spikes have several negative consequences, one of which e.g. is the relatively large dimensioning of the output capacitor. In addition, these current spikes result in high noise radiation. When the input voltage of the converter is furnished by a battery, e.g. a Li ion cell the high spike currents result in a reduction in the life of the battery. On top of this, a heavy ripple in the output voltage is produced by these high current spikes.
It is thus an object of the present invention to improve a DC/DC converter as cited at the outset operating on the charge pump principle so that the output current spikes of the charge pump and the output voltage ripple are greatly reduced.
This object is achieved by a DC/DC converter as cited at the outset which is characterized by a regulator circuit receiving the control signal of the comparator and converting it into a signal characterizing the momentary ON/OFF duration ratio of the charge pump with which it controls the ON resistance of one of the switches so that the ON/OFF duration ratio of the charge pump can be regulated to a predetermined design value, and by a further controllable switch connected in parallel with the switch whose ON resistance is controlled and which is controlled by the control circuit correspondingly thereto.